PROJECT SUMMARY Skin-penetrating nematodes, including the human-parasitic threadworm Strongyloides stercoralis, infect nearly one billion people worldwide and are a major source of morbidity in low-resource settings. Infections can cause chronic gastrointestinal distress, stunted growth and cognitive impairment in children, and even death in the case of S. stercoralis infection. S. stercoralis has a complex life cycle that includes a parasitic generation inside the host and a free-living generation outside the host. In previous work, we showed that the detection of carbon dioxide (CO2) and host-emitted odorants is important for multiple stages of the S. stercoralis life cycle. Moreover, we showed that CO2 and many host-emitted odorants elicit life-stage-specific behavioral responses, such that the chemosensory preferences of the infective larvae are distinct from those of the non-infective life stages. However, the neural mechanisms that mediate these chemosensory responses have not yet been investigated. Here, we propose to elucidate the molecular, cellular, and circuit mechanisms of chemosensation in S. stercoralis. In Aim 1, we will elucidate the neural mechanisms that mediate CO2 response in S. stercoralis. We will also investigate how CO2 microcircuit function is modulated across life stages to generate life-stage- specific responses to CO2. In Aim 2, we will elucidate the neural mechanisms that mediate responses to host- emitted odorants in S. stercoralis. We will also investigate how olfactory microcircuit function is modulated across life stages. In Aim 3, we will address the molecular mechanisms of chemosensation. We will identify genes and signaling pathways that mediate responses to CO2 and host-emitted odorants in S. stercoralis. We will also identify molecular mechanisms that contribute to parasite-specific and life-stage-specific chemosensory responses. Taken together, our results will provide key insights into the chemosensory mechanisms that underlie the complex interactions of parasitic nematodes with their human hosts.